This application discloses a channel estimation method and apparatus, a communications system, pertains to the field of communications technologies. The method is applied to an AP, includes: obtaining communication quality between the AP and a STA; determining, based on the communication quality, a quantity of channel estimations that need to be performed in a current round of TSCE; and obtaining channel state information CSI between the AP and the STA, where the CSI is determined based on a channel estimation result obtained after the current round of TSCE is performed based on the determined quantity of channel estimations. In this application, the AP adjusts, in real time based on the communication quality, a quantity of channel estimations that need to be performed in each round of TSCE, to resolve a problem of relatively low reliability of channel estimation in a related technology.